1. Field of the Invention
The present invention relates to a noise filter and a high frequency transmitter using the same. More specifically, the present invention relates to a noise filter formed by a microstrip line provided on a substrate and a high frequency transmitter provided with such a noise filter on the output side of a transmission power ampflifier.
2. Description of the Background Art
In recent years, rapid progress is made in the market for the radio communication using high frequencies in many systems such as broadcasting satellites and communications satellites. At the same time, the demand is increasing day by day for two-way communication according to the development of the Internet. In the two-way communication in the satellite communication, reception is implemented by LNB (Low Noise Block Down Converter) as is conventionally done, while transmission is implemented by newly using a high frequency transmitter.
FIG. 8 is a block diagram representing an arrangement of a conventional high frequency transmitter, FIG. 9 is a diagram illustrating the shape of a reception band noise filter used in the conventional high frequency transmitter, and FIG. 10 is a simulation result for a conventional reception band noise filter.
Now, a high frequency transmitter of a conventional example will be described with reference to FIGS. 8 to 10. An IF (intermediate frequency) signal input to the high frequency transmitter shown in FIG. 8 is input to a mixer circuit 2 after having its gain ensured by an IF amplifier 1. In mixer circuit 2, a local oscillation signal from a local oscillation circuit 3 and the IF signal are mixed, and the IF signal is frequency-converted into a high frequency signal. The high frequency signal output from mixer circuit 2, after passing through a band-pass filter 4 that attenuates the spurious that is generated in mixer circuit 2, obtains a large gain from a circuit configured by three high frequency amplifiers 5, 6, and 7.
The output from high frequency amplifier 7 is input via a band-pass filter 8 that attenuates the amplified spurious to a high frequency amplifier 9, and together with a succeeding driver amplifier 10, more gain is earned. The output of driver amplifier 10 is input via a reception band noise filter 11 that limits the noise level of the reception frequency band down to a negligible level to a power amplifier 12, and becomes a high power signal required for transmission to a satellite. The high frequency signal output from power amplifier 12 passes via a reception band noise filter 13 that once again attenuates the noise level of the reception frequency band that has risen from the thermal noise level due to the gain of power amplifier 12 and an isolator 14 for ensuring isolation between an RF output and reception band noise filter 13 and is output from the high frequency transmitter (not shown).
Now, as shown in FIG. 9, for reception band noise filters 11 and 13, a microstrip filter is generally employed which is formed by a main microstrip line 15, one end of which has an input signal supplied thereto and the other end of which outputs a signal, and three sub-microstrip lines 16, 17, and 18 that are disposed together one by one such that they run orthogonal to main microstrip line 15.
The reason for employing a filter of such a shape lies in that it allows large attenuation to be obtained in relation to the reception frequency band, while at the same time, the loss in the transmission frequency band can be limited to as low as 1 dB. When the loss is great in the transmission frequency band of reception band noise filter 13 disposed downstream to power amplifier 12, there is a need to select a power amplifier of the type having large output power (the type having large saturation power) for power amplifier 12. The power amplifier with large output power also involves high power consumption and greater heat generation so that the shape of the overall high frequency transmitter must be enlarged for the purpose of heat radiation, which, as a result, goes against the conditions such as compactness and low power consumption for its widespread use. Therefore, a filter of the shape as shown in FIG. 9 that has small loss in the transmission frequency band is employed.
The signal pass characteristic of the filter shown in FIG. 9 is indicated by the simulation result shown in FIG. 10. As shown in FIG. 10, the filter is optimized such that a signal can pass through at a transmission frequency of 14 to 14.5 GHz and attenuates at a reception frequency of 10.95 to 12.75 GHz, and the loss of the transmission frequency is about 1 dB and the attenuation of the reception frequency obtained is at least 25 dB.
When the noise level of the reception frequency band that is input to power amplifier 12 is lowered to the thermal noise level (xe2x88x92173.5 dBm/Hz (25xc2x0 C.)) due to the attenuation of band-pass filters 4 and 8 and reception band noise filter 11, and when the small signal gain of power amplifier 12 is 20 dB and the noise figure is 7 dB, the noise level of the reception frequency band output from power amplifier 12 rises as high as xe2x88x92173.5+20+7=xe2x88x92146.5 dBm/Hz. This level, however, would be limited to xe2x88x92146.5xe2x88x9225=xe2x88x92171.5 dBm/Hz by being input into reception band noise filter 13 having the shape and characteristic of FIGS. 9 and 10. The specifications of the reception band noise level of a common high frequency transmitter is about xe2x88x92165 dBm/Hz or below, and it can be recognized that the specifications are satisfied by the effect of reception band noise filter 13.
The recent development trends involve movements toward widely spreading high frequency transmitters among ordinary households as well as achieving lower cost and compactness, and a high gain type power amplifier with a small signal gain of about 35 dB is increasingly being adopted. By employing a high gain type power amplifier, components such as a driver amplifier and a high frequency amplifier become unnecessary, which contributes to cost and size reduction.
The increase in the small signal gain of the power amplifier, however, leads to greater increase in the noise level of the reception frequency band, which leads to the problem of the specifications of the reception band noise level not being satisfied.
Let us assume a case where a power amplifier 12 shown in FIG. 8 is replaced by a power amplifier 19 having a small signal gain of 35 dB. When the noise level of the reception frequency band input to power amplifier 19 is lowered to the thermal noise level (xe2x88x92173.5 dBm/Hz (25xc2x0)), with the small signal gain of power amplifier 19 being 35 dB and the noise figure being 7 dB, the noise level of the reception frequency band output from power amplifier 19 rises as high as xe2x88x92173.5+35+7=xe2x88x92131.5 dBm/Hz. By inputting a signal to reception band noise filter 13 having the shape and characteristic of FIGS. 9 and 10, the level can be limited to xe2x88x92131.5xe2x88x9225=xe2x88x92156.5 dBm/Hz; however, this level does not satisfy the specifications of the reception band noise level of a general high frequency transmitter of about xe2x88x92165 dBm/Hz.
Thus, the principal object of the present invention is to provide a noise filter having large attenuation in the reception frequency band and a high frequency transmitter using the same.
In short, according to the present invention, a noise filter formed by a microstrip line disposed on a substrate includes a main microstrip line, one end of which has an input signal supplied thereto and other end of which outputs a signal, and at least first to fifth sub-microstrip lines disposed together one by one such that they intersect with the main microstrip line and their lengths from the intersections to their respective ends vary.
Thus, according to the present invention, the attenuation can be made large in the reception frequency band by disposing at least first to fifth sub-microstrip lines such that they intersect with the main microstrip line.
Preferably, the first to fifth sub-microstrip lines are each formed in a generally rectangular shape.
Consequently, the frequency selectivity of the filter improves, and the frequency resolution can be enhanced.
Preferably, the line widths of the first to fifth sub-microstrip lines are all formed to have the same prescribed length.
Consequently, the Q-values of all sub-microstrip lines can be made the same. By optimizing the line width according to the frequency bandwidth of the attenuation band or the pass bandwidth of a signal required, a filter can be provided that has large attenuation in the attenuation band, excellent flatness in the pass band, and small pass loss.
More preferably, the sub-microstrip lines are each disposed all at the same prescribed intervals and generally parallel to one another so that it becomes possible to prevent high frequency coupling between the sub-microstrip lines and to prevent degradation in characteristics as a filter.
More preferably, of the first to fifth sub-microstrip lines, the first, third, and fifth sub-microstrip lines are disposed such that they are shifted in one direction generally orthogonal to the main microstrip line and the second and fourth sub-microstrip lines are disposed such that they are shifted in other direction generally orthogonal to the main microstrip line.
As a result, coupling between adjacent sub-microstrip lines can be prevented, and the degradation in characteristics as a filter can be prevented.
More preferably, with the third sub-microstrip line in the center, the first and second sub-microstrip lines and the fourth and fifth sub-microstrip lines are disposed in line symmetry.
Consequently, the first and fourth sub-microstrip lines would have the same length and the second and fifth sub-microstrip lines would have the same length, and it becomes possible to obtain an even larger attenuation in the attenuation band with the resonance points overlapping at the same frequency.
More preferably, with respective intersections of the main microstrip line and the first and fifth sub-microstrip lines serving as boundaries, the line width is set such that the portion between the first sub-microstrip line and the fifth sub-microstrip line becomes greater in width than the portions between the intersections and the one end and the other end.
Thus, inductivity of the main microstrip line can be limited, and the impedance in high frequency band is reduced, and the insertion loss in the pass band of the noise filter can be limited.
More preferably, with respective intersections of the main microstrip line and the first to fifth sub-microstrip lines serving as boundaries, respective line widths are set such that they become greater closer to the third sub-microstrip line away from the one end and the other end portion.
As a result, inductivity of the main microstrip line can be limited, and the impedance in high frequency band is reduced. At the same time, impedance mismatch can be alleviated in the discontinuous portions of the line width created by making the line width greater away from one end and the other end portion, and the insertion loss in the pass band of the noise filter can be limited.
More preferably, the line length of the third sub-microstrip line can be changed so as to set the pass frequency bandwidth to the desired band.
According to another aspect of the present invention, a noise filter formed by a microstrip line disposed on a substrate is connected to the output side of a transmission power amplifier for amplifying a high frequency transmission signal, and the noise filter includes a main microstrip line, one end of which has an input signal supplied thereto and other end of which outputs a signal, and at least first to fifth sub-microstrip lines disposed together one by one such that they intersect with the main microstrip line and their lengths from the intersections to their respective ends vary.
The noise filter thus configured has small insertion loss in the pass band, can ensure output VSWR (Voltage Standing Wave Ratio) characteristic of the transmission output without an isolator, and can omit the corresponding amount for the insertion loss of the isolator so that the output power of the transmission power amplifier would suffer little burden, and advantages can be gained in terms of heat radiation and chassis shape.
Moreover, the line length of a sub-microstrip line of the noise filter can be adjusted so as to improve the output return loss of the high frequency transmitter.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.